Change project root package (Avoid "Validation Error")

CUBA 6.2.8 (Studio 2.2.4)

When I try to create/edit a service, I get the following error message:
“Validation error
Package should be inside root package:”

I guess this error message refers to the “Root package” of my project which is empty. However, the root package can not be changed from within CUBA studio as the text field is read only.

This is the path of the Root package property in CUBA Studio:
Project Properties -> Edit -> Main -> Root package

My (only) entity lies in a package starting with “de.mycompany”.

How can I change the root package in order to avoid the validation error?

Strange enough, on another computer the package becomes visible when loading the same project.
This had neither something to do with the browser (Firefox vs. Chrome) nor something with the Gradle version (3.4.1 vs 2.13).

In the studio.log, I found an error, but I am not sure whether it has something to do with the missing root package:

2017-07-18 23:00:17,961 ERROR [org.atmosphere.container.Jetty9WebSocketHandler] {} Timeout on Read
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(
at org.eclipse.jetty.util.thread.QueuedThreadPool$

I removed the project from the recent projects list and then imported it project again - that solved the problem!